Add a simple json module to Transcrypt

[nandoflorestan]

Transcrypt is awesome, thank you!

I want to use a single validation library on both sides of the wire. I have been trying to make a few Python validation libraries to work in the client, too. This can solve the problem of keeping validation code in sync. But in order to do so, I need the json module.

Most Python validation libraries use the json module. It's probably impossible to implement the subtleties of the json module, but 99% of its usage is simple, and this we can do in very little code.

Let me know about any problems you see, of course.

[JennaSys]

May be related to #868